1. Field of the Invention
This invention pertains generally to a device for accurate estimation of an underwater towed vehicle's position, and more particularly to the making of an estimate of an underwater towed vehicle's position independent of off-board sensors and horizontal range from the towing vehicle.
2. Description of the Prior Art
Underwater towed vehicles are in common use for surveying and location of objects in the ocean due to their relatively low operational cost. To be useful however, the position of the towed vehicle must be known so that the data collected by the towed vehicle's sensors may be geo-rectified. Due to currents in the water and the cantilever in the towing cable, a towed vehicle's position relative to the towing vessel is not easily determined. Since the ocean is opaque to high-frequency electromagnetic signals, the global positioning system (GPS) cannot be used for tow vehicle position.
Current methods for determining the position of a towed vehicle include the lay-back model (See, FIG. 1), the long-base line (LBL) (See, FIG. 2), the short base line (SBL) (See, FIG. 3), and the inverted SBL systems. With the lay-back model method the towed vehicle is assumed to be directly behind the towing vehicle and the lay-back (distance from the towing vehicle) is assumed to be a fixed multiple of the tow vehicle's depth. Alternatively, the amount of cable paid out is assumed to be the slant range to the tow vehicle. The latter method does not take into account the cable's cantilever. Neither method takes into account the effects of local currents on the cables' shape and position. The accuracy of these methods will in general not support precise positioning of the towed vehicle.
The LBL method uses a series of acoustic transponders to localize the tow-vehicle by measuring the time delays between the tow-vehicle and the transponders. These transponders may be bottom mounted or located on the surface, such as shown in U.S. Pat. No. 5,119,341. While these systems are very accurate, their deployment is a time consuming and expensive operation. Furthermore, the LBL method requires that the system be redeployed to each operational area.
The SBL method uses time delay and measured arrival angle of an acoustic signal from the tow-vehicle to the towing vehicle to compute position. These systems are fairly accurate but their resolution is range dependent. Typically, in order to achieve sufficient accuracy, a SBL system is deployed over the tow-vehicle, requiring a second surface craft--an obvious economic disadvantage.
With the inverted SBL method, the directional acoustic receiver is mounted on the towed vehicle instead of the towing vehicle. As a consequence the range dependent accuracy of this system is a problem for very deep tows. SBL and inverted SBL systems cannot be readily used in applications with helicopter towed systems.